1. Field of the Invention
This invention relates to bolting bar materials suitable for the manufacture of bolts having an excellent toughness of 7T grade (70.about.80 kgf/mm.sup.2) to 8T grade (80.about.90 kgf/mm.sup.2) and a method of producting the same.
2. Description of the Prior Art
Heretofore, bolts used for assembling mechanical parts are usually manufactured by cold molding a bolting material and subjecting it to a heat treatment so as to provide a required strength, which is performed, for example, according to a flow sheet as shown in FIG. 1.
In the above conventional process, however, the secondary working step is lengthened, so that the manufacture of the bolt takes a long time and the production cost becomes increased.
In order to solve the above drawbacks, there have been developed so-called non-heat treated steels, which allow to omit heat treatments such as annealing, quenching. tempering and the like. Among them, some steels have been attempted to use as a material for the manufacture of 7T-8T grade bolts, but have not yet been put into practical use. Because, the bolt made from such a non-heat treated steel is low in the toughness. That is, the non-heat treated steel certainly has a tensile strength of 70.about.90 kgf/mm.sup.2 as it is rolled, but the bolt made therefrom frequently causes the fracture at the junction of head, for example, when it is subjected to a wedge tensile test as an actual test.